A honeycomb sandwich panel as disclosed for example in Patent Literature 1 listed below is sometimes used for the floors of the cabin and the luggage compartment of an aircraft. This honeycomb sandwich panel is a panel obtained by fixedly attaching a honeycomb layer which is a collection of hexagonal cells or hexagonal prism cells and honeycomb skin layers which are laid on the upper and lower surfaces of the honeycomb layer. The honeycomb skin layers are made for example of carbon fiber-reinforced plastic (CFRP) or the like.
The honeycomb sandwich panel is light and rigid. When it is used, its thickness is often reduced as much as possible while its strength is taken into account to thereby achieve a further weight reduction.